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Sedna shrinks after new measurement

Trans-Neptunian object way smaller than Pluto may carry water ice

By Simon Sharwood, 5 Apr 2012

Dwarf planet wannabe Sedna is considerably smaller than previously assumed, according to a new study of Trans-Neptunian Objects (TNOs).

Once the most distant object observed in the solar system, Sedna was thought to have a diameter of around 1700km. But a new paper, dubbed "TNOs are Cool": A survey of the trans-Neptunian region” suggests the object has a diameter of just 995±80 km.

This estimate comes from new data drawn from Photodetector Array Camera and Spectrometer (PACS) on the European Space Agency's Herschel Space Observatory. The new size estimate was derived by measuring Sedna's predicted rotation period.

The paper says the PACS instrument detected a greater albedo than had previously been measured, suggesting the presence of water ice on the remote rock.

The study also considered the evocatively-named 2010 EK, another TNO that had previously been assumed to have a diameter of about 770km. The new study suggests it is only 470km or so around. No explanation is offered for the more optimistic previous estimates.

It's not hard to imagine that these new measurements will fuel ongoing definition debates about what constitutes a dwarf planet, as previous estimates suggested Sedna was abou 80% the size of Pluto. ®